In flip-chip device mounting methods, small metal projections, or microbumps, are provided on the electrode contacts of a semiconductor device, or on a circuit to which the device is to be connected. The semiconductor device is aligned with the conductor pattern on the circuit and is fixed to the circuit by soldering or by a curable resin adhesive. See for example U.S. Pat. No. 4,749,120 issued Jun. 7, 1988 to Hatada.
In one type of apparatus, an LED (light emitting diode) print head is mounted on a glass substrate having a photographic mask, by the flip-chip mounting technique. See U.S. Pat. application Ser. No. 375,154 by Contois et al, now U.S. Pat. No. 4,980,701. The glass substrate may also incorporate a lens array, such as a sel-foc lens for the LED print head. The flip-chip mounting method permits precise registration of the surface of the LED die with respect to such a lens array.
It is desirable that the circuit conductors be as thick as practical for low-resistance, high-frequency operation.
It is also desirable that the microbumps be deformable to allow for slight variations in height, and microcircuit flatness during bonding operations. Etched aluminum patterned with a photoresist mask has been traditionally employed as a conductor for the circuits employed in flip-chip mounting and solder bumps have been used for the microbumps. A problem with formation of aluminum conductors by patterning with photoresist is that the film thickness is limited due to the durability of the resist. A further problem is that the solder bumps are not as soft or ductile as would be desirable for compensating for the differences in feature height by deformation of the bumps.